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Abstract:

A luminaire for illuminating a target area (34) by retroreflection from a
reflector (30), comprising a light-emitting diode module (12) having at
least one light-emitting diode; (14) and a cooling device (10) for the
light-emitting diode module (12), the cooling device including as coolant
a liquid (26) that is transparent to the light of the light-emitting
diode module and a transparent coolant container (20) for accommodating
the coolant; (26) wherein the coolant container (20) comprises a first
(22) and a second transparent wall (24) between which the coolant (26) is
located and which extend substantially perpendicularly to the optical
axis (36) of the reflector (30) and have a surface such that in an
assembled state in which the light-emitting diode module (12) and the
cooling device (10) are connected to the reflector (30), at least 90% of
the radiation of the light-emitting diode module (12) which is reflected
from the reflector (30) and which reaches the target area (34) passes
through the coolant container (20).

Claims:

1. A luminaire for illuminating a target area by retroreflection from a
reflector, comprising: a light-emitting diode module having at least one
light-emitting diode; and a cooling device for the light-emitting diode
module, the cooling device including as coolant a liquid that is
transparent to the light of the light-emitting diode module and a
transparent coolant container for accommodating the coolant; wherein the
coolant container comprises a first and a second transparent wall between
which the coolant is located and which extend substantially
perpendicularly to the optical axis of the reflector and have a surface
such that in an assembled state in which the light-emitting diode module
and the cooling device are connected to the reflector, at least 90% of
the radiation of the light-emitting diode module which is reflected from
the reflector and which reaches the target area passes through the
coolant container.

2. The luminaire as claimed in claim 1, wherein the first and the second
wall of the coolant container have a surface such that all of the
radiation of the light-emitting diode module which is reflected from the
reflector and which reaches the target area passes through the coolant
container.

3. The luminaire as claimed in claim 1, wherein the at least one
light-emitting diode is embodied in such a way that it emits radiation at
most into one hemisphere.

4. The luminaire as claimed in claim 1, wherein the cooling device
additionally serves as a retainer for the light-emitting diode module.

5. The luminaire as claimed in claim 4, wherein the light-emitting diode
module includes a thermally conducting element which is coupled to the at
least one light-emitting diode and which is recessed into the coolant
container such that it is in contact with the coolant.

6. The luminaire as claimed in claim 5, wherein the thermally conducting
element is embodied as a solid cylinder having fins projecting outward
therefrom.

7. The luminaire as claimed in claim 5, wherein the light-emitting diode
module includes a printed circuit board on the top side of which one or
more light-emitting diodes are mounted and the bottom side of which is
connected to the thermally conducting element, the dimension of the
thermally conducting element in a direction perpendicular to the optical
axis of the light-emitting diode module being less than or equal to the
corresponding dimension of the printed circuit board.

8. The luminaire as claimed in claim 1, wherein the first wall of the
coolant container has conductive coatings for the electrical contacting
of the light-emitting diode module.

9. The luminaire as claimed in claim 1, wherein the first and the second
wall of the coolant container are embodied as plane-parallel plates.

10. The luminaire as claimed in claim 1, wherein the outside surface of
the first wall and/or the second wall of the coolant container is
embodied as curved in order to realize an optical function.

11. The luminaire as claimed in claim 10, wherein a two-dimensional lens
array is embodied on the outside surface of the first and/or the second
wall of the coolant container.

12. The luminaire as claimed in claim 7, wherein the bottom side of said
printed circuit board is connected to said thermally conducting element
by a thermally conductive material.

Description:

TECHNICAL FIELD

[0001] The invention relates to a luminaire for illuminating a target area
by means of retroreflection from a reflector, said luminaire comprising a
light-emitting diode module having at least one light-emitting diode
(LED), as well as a cooling device for the light-emitting diode module,
the cooling device including as coolant a liquid that is transparent to
the light of the light-emitting diode module and a transparent coolant
container for accommodating the coolant.

BACKGROUND ART

[0002] An illumination system which uses a light source of said type is
known from US 2007/0253733 A1. The cited document describes the use of
the illumination system for a fluorescence microscope. An LED light
source is positioned at a focal point of an elliptical mirror and emits
its radiation into a hemisphere facing toward the mirror. The mirror
reflects the incident radiation and focuses it onto a downstream optical
system. Due to the retroreflection from the reflector, i.e. reflection of
beams having an angle of incidence of less than 45°, it is
impossible to prevent the LED light source and the mechanical structures
required for retaining it from themselves obstructing the reflected
light.

[0003] Because the light yield of a LED decreases with increasing
temperature it is necessary to ensure that the heat generated during its
operation will be dissipated in order to minimize the heating-up of the
LED during operation. If a LED module is arranged at the focal point of a
backward-reflecting reflector, the module cannot be cooled by an
otherwise conventional heatsink because the latter would shadow an even
greater part of the light reflected from the reflector. Instead of this,
the heat must be conducted to the outside by means of the retainers. Even
if the latter can be implemented in a very compact design in order to
keep losses of the reflected light to an absolute minimum, they take up
more room than a LED on its own and consequently lead to a shadowing of
the reflected light. Even if the retainers are embodied as transparent,
the reflected radiation incident thereon is affected and the optical
efficiency of the arrangement reduced.

SUMMARY OF THE INVENTION

[0004] The object of the present invention is to provide a luminaire of
the generic type in which a high level of efficiency in terms of light
transmission and heat dissipation of the LED module is made possible
while at the same time aiming to minimize the generation of artifacts
such as shadows.

[0005] This object is achieved by means of a luminaire having the features
of claim 1.

[0006] Particularly advantageous embodiments can be found in the dependent
claims.

[0007] The present invention is based on the knowledge that liquid cooling
of a LED module can be designed in such a way that the light emitted by
the LED and reflected by the reflector is influenced in a defined manner
when it impinges on the coolant container. Toward that end the coolant
container has a first and a second transparent wall between which the
coolant is located and which extend substantially perpendicularly to the
optical axis of the reflector and have a surface such that in an
assembled state in which the LED module is coupled to the reflector
virtually all, i.e. at least 90%, of the radiation of the light-emitting
diode module which is reflected by the reflector and which reaches the
target area passes through the coolant container.

[0008] The coolant container therefore has two walls parallel to each
other and covers the entire cross-section of the radiation emanating from
the light-emitting diode module and reflected by the reflector. The walls
of the coolant container can be made of glass or plastic. The coolant can
be water, for example, whose refractive index of 1.33 differs only
slightly from that of glasses or transparent plastics having a low
refractive index (approx. 1.5), or transparent oil. In the choice of the
coolant and the material for the coolant container it is in any case
advantageous if the refractive index of the first and the second wall and
that of the coolant are so similar that the reflection losses at the
interfaces between the walls and the coolant are very small. The first
and the second wall of the coolant container can be rectangular, in
particular square, or their outline can be matched to the outline of the
reflector.

[0009] It is of advantage in this case if the outer edge of the coolant
container lies outside of the area into which the radiation emanating
from the luminaire is reflected in order to rule out any detrimental
effect on the reflected radiation. In that case it is also irrelevant
whether the edge itself is transparent or not. In the ideal case all of
the radiation of the light-emitting diode module which is reflected by
the reflector and which reaches the target area passes through the
coolant container.

[0010] Preferably the LED module is embodied such that the at least one
light-emitting diode emits light at most into one hemisphere. In this way
the LED module can be arranged such that all of the emitted light is
incident on the reflector and is reflected by the latter in the direction
of the target area, with the result that the light reaching the target
area has defined characteristics.

[0011] According to a preferred embodiment of the invention the cooling
device additionally serves as a retainer for the light-emitting diode
module. As a result two functions are combined in one component, while
shadowing caused by an additional retainer is avoided.

[0012] According to a particularly preferred embodiment of the invention
the light-emitting diode module includes a thermally conducting element
which is coupled to the light-emitting diode and is recessed into the
coolant container such that it is in contact with the coolant. In this
way the LED module is secured to the cooling device and at the same time
effective dissipation of the heat being generated during operation is
ensured. A particularly efficient transfer of heat from the thermally
conducting element to the coolant can be achieved for example if the
thermally conducting element is embodied as a solid cylinder having fins
projecting outward therefrom or has holes through which the coolant
flows. In addition the thermally conducting element can have a rough or
structured surface.

[0013] According to another particularly preferred embodiment of the
invention the light-emitting diode module includes a printed circuit
board on the top side of which one or more light-emitting diodes are
mounted and the bottom side of which is connected to the thermally
conducting element in particular by means of a thermally conductive
material, the dimension of the thermally conducting element in a
direction perpendicular to the optical axis of the light-emitting diode
module being less than or equal to the corresponding dimension of the
printed circuit board. In this way only the shadowing at the LED module
reduces the amount of light reaching the target area.

[0014] According to a further preferred embodiment of the invention the
first wall of the coolant container, i.e. the wall which in the assembled
state faces toward the reflector, has conductive coatings for the
electrical contacting of the light-emitting diode module, which
conductive coatings can be implemented as transparent.

[0015] According to another preferred embodiment of the invention the
first and the second wall of the coolant container are embodied as
plane-parallel plates. In this way the radiation passing through the
coolant container is affected to an absolute minimum.

[0016] According to a further preferred embodiment of the invention the
outside surface of the first and/or the second wall of the coolant
container is embodied as curved in order to realize a specific optical
function. This enables the coolant container simultaneously to assume the
function of an optical element, for example a lens, as a result of which
additional components and consequently costs can be saved.

[0017] Particularly preferably, a two-dimensional lens array is embodied
on the outside of the first and/or the second wall of the coolant
container. In particular the coolant container can have the form of a
honeycomb condenser, thereby effecting a homogenization of the radiation
passing through it.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The invention shall be explained in more detail below with
reference to exemplary embodiments and the accompanying figures, in
which:

[0019] FIG. 1 shows a schematic sectional view of a first exemplary
embodiment of the luminaire according to the invention; and

[0020] FIG. 2 shows a schematic sectional view of a second exemplary
embodiment of the luminaire according to the invention.

PREFERRED EMBODIMENT OF THE INVENTION

[0021] Parts corresponding to one another in the figures are labeled with
the same reference signs. The parts depicted and the proportions of the
parts relative to one another should not be regarded as true to scale.

[0022] FIG. 1 shows a first exemplary embodiment of a luminaire according
to the invention. The luminaire comprises a LED module 12, which is
combined with a reflector 30 in such a way that the light emitted by the
LED or LEDs 14 is reflected from the reflector 30 in a backward
direction, and a cooling device 10 for cooling the LED module and
simultaneously serving as a retainer for the LED module 12. The LED
module 12 comprises one or more LEDs 14 represented by a semicircle, and
a thermally conducting element 16. A LED module 12 can also have a
plurality of LEDs 14 instead of just one.

[0023] The LED or LEDs 14 is or are mounted on a printed circuit board on
the bottom side of which the thermally conducting element 16 is arranged.
Preferably the thermally conducting element 16 is a copper block. Its
outline is matched to the shape of the printed circuit board, its size in
a direction perpendicular to the optical axis of the LED module being
less than or equal to the corresponding size of the printed circuit
board. In this way the light loss due to shadowing is reduced to a
minimum. The thermally conducting element 16 is embodied in particular as
a solid cylinder and has fins or ribs projecting outward therefrom in
order to enable the heat absorbed during operation to be released
particularly effectively to the cooling device 10.

[0024] The cooling device comprises a flow-through coolant container 20
having a first wall 22 and a second wall 24 between which is contained a
coolant 26. The thermally conducting element 16 is recessed into the
coolant container 20 through a corresponding opening in the first wall 22
such that it is in contact with the coolant 26 and that the LED module 12
is anchored to the coolant container 20 thereby. For the electrical
contacting of the LED module 12 there is embodied on the exterior of the
first wall 22 a transparent conductive coating (not shown in the figure)
which is connected to the printed circuit board of the LED module 12.
Alternatively, however, the electrical contacting can also be realized by
means of thin wires. As can be seen from FIG. 1, at the top and bottom
edge the coolant container 20 has feed lines 27, 28 through which the
coolant 26 can flow into and out of the coolant container 20. For
example, the coolant 26 flows into the coolant container 20 through the
top feed line 27 and out of the coolant container 20 again through the
bottom feed line 28, such that coolant circulates around the thermally
conducting element 16. It would, however, also be conceivable for the
coolant container to be sealed after being filled with coolant, and for
no active circulation of coolant to take place during operation. A
transparent liquid is used as the coolant 26. This can be water, whose
refractive index of 1.33 is not markedly different from the refractive
index of transparent materials that are considered suitable for the walls
of the coolant container 20, such as glass with a refractive index of
1.41 for example. The walls 22 and 24 of the coolant container 20 are
aligned parallel to each other and perpendicular to the optical axis 36
of the LED module 12. Their surface area is greater than that of the beam
of rays reflected by the reflector 30.

[0025] In order to illustrate the beam path, the two edges of the
radiation emitted by the LED module 12 are depicted in FIG. 1. The walls
22 and 24 are matched rectangularly or in terms of their outline to the
reflector 30. They are connected to each other at their edges by suitable
means, in particular by sidewalls. Said edge regions are preferably
located outside of the area of the reflected radiation in order to avoid
interfering with the reflected radiation. The inside surfaces of the
walls of the coolant container 20 are embodied as smooth in order to
avoid any turbulence being generated in the coolant 26 at the walls as it
flows through. According to the first exemplary embodiment of the
invention shown in FIG. 1, the outside surfaces in particular of the
first wall 22 and the second wall 24 of the coolant container 20 are also
embodied as smooth, with the result that the two walls 22 and 24 are
plane-parallel plates. In this way the entire coolant container 2Q acts
on the incident radiation as a plane-parallel plate.

[0026] FIG. 2 shows a second exemplary embodiment of the luminaire
according to the invention. The exemplary embodiment according to FIG. 2
is characterized in that in addition to the function of cooling and
retaining the LED module 12 the coolant container 20 also assumes an
optical function. In other respects the cooling device 10 according to
the second exemplary embodiment corresponds to the description given in
connection with the first exemplary embodiment according to FIG. 1. The
coolant container 20 shown in FIG. 2 is embodied as a honeycomb
condenser. Toward that end lens arrays are disposed on the outside
surfaces of the first wall 22 and the second wall 24 of the coolant
container 20. Homogenization of the radiation passing through the coolant
container 20 is effected as a result. Viewed in the propagation direction
of the radiation, a Fresnel lens 32 is additionally provided after the
coolant container 20 for focusing the radiation onto the target area 34.
Integrating the optical function of a honeycomb condenser into the
coolant container 20 enables savings to be made not only in terms of
space but also in terms of material and consequently costs. Other desired
optical functions can also be realized in similar fashion by suitable
embodiment of the walls 22 and 24 of the coolant container 20.

[0027] In both exemplary embodiments the coolant container 20 is arranged
in the luminaire in such a way that the LED module 12, whose LED or LEDs
emits or emit light at most into one hemisphere, is located close to the
focal point of the reflector. The reflector 30 accordingly collects all
of the LED radiation and can transmit same through the coolant container.
In particular conic section figures, such as paraboloids or ellipsoids,
are employed as reflectors. Using a reflector in a retroreflective manner
therefore results in effective exploitation of the radiation in a simple
design.

[0028] The luminaire according to the invention can be designed in such a
way that it can be installed in existing lamps having a reflector, such
that a retrofit of prior art halogen lamp solutions is possible.
Preferred applications for the solution according to the invention are
medical luminaires, small-etendue applications such as projectors, or
high axis light intensity applications such as headlights. It is
particularly efficient and cost-effective when a large-diameter optics
system is necessary.